1. Field of the Invention
This invention relates in general to gate valves, and in particular to a seat seal which will block flow only in one direction.
2. Description of the Prior Art
A typical gate valve used in connection with oil and gas production has a body with a flow passage extending through it. The flow passage intersects a central cavity. Seat rings are placed in counterbores formed in the flow passage at the intersection of the flow passage with the cavity. A gate will be moved past the seats between open and closed positions to cause sealing.
The seats have seals which seal the seat to the counterbore of the flow passage. These seals prevent the entry of fluid from the central cavity or chamber of the body to the downstream flow passage. When the gate is opened, the seals perform no function. When the gate is closed, fluid will flow past the upstream seat into the chamber or cavity of the body. The fluid pressure in the chamber is sealed by the seal of the downstream seat.
A typical seal is U-shaped, having two legs which extend axially from a base. The base will bear against a counterbore shoulder, while the legs seal between the counterbore cylindrical wall and a cylindrical wall of the seat. A standoff ring inserts into the recess between the legs and bears against a shoulder on the seat that opposes the counterbore shoulder. The legs will seal when the pressure is outward directed across the seal, but are not intended to seal when pressure is inward directed across the seal.
Some operators periodically test the valve while installed in the field. A typical test involves closing the gate, then applying a surge of test pressure to the downstream side of the flow passage. Because the pressure is inward directed against the downstream seal, it is intended that the pressurized fluid flow past this downstream seal, into the cavity and act against the upstream seal. The upstream seal, having pressure acting against it in an outward direction, will seal.
One problem from applying this surge of test pressure is that the downstream seal may not always allow the pressure to flow by, rather it may seal itself. As the pressure is directed in the wrong direction against the downstream seal during a pressure test, the sealing is accomplished only by extreme deformation of the downstream seal, with the downstream seal and standoff ring jamming toward the cavity. Consequently, while the test indicates that the valve is sealing, in actuality, the test is harming the downstream seal. Subsequently, when the fluid is flowing in a normal upstream to downstream direction and the gate is then closed, the downstream seal may not seal as it should.